1. Field of the Invention
The present invention relates to an isolating device and more particularly to such a device for isolating fluctuations in the actual drive torque of a drive shaft of a machine about a nominal drive torque level.
2. Description of the Prior Art
Such isolating devices are well known and are often mounted on the crankshaft of the internal combustion engine of a motor vehicle so that it can drive a driven component such as an alternator, a fan or other piece of equipment. Devices of this type are often fitted with a torsional vibration damper in the form of an annular inertia member mounted on an elastomeric material.
One example of an isolating device of this kind is disclosed in the applicant's co-pending PCT application No. GB 96/00371. This application describes a drive device comprising a primary element connected to the drive shaft and a secondary element having a contoured pulley rim to which a V-belt drive is attached. The belt transmits power from the drive shaft to a driven component. The primary and secondary elements are interconnected by a torsionally flexible elastic ring which is loaded in shear and effectively absorbs rotational fluctuations in the motion of the driving shaft so that they are not transmitted to the driven component. Torsional vibrations of the shaft to which the device is attached are damped by means of an inertia ring connected to the primary element by means of an elastic element.
One disadvantage of a device of this type is that the primary and secondary elements are subject to a relatively large relative rotational displacement during initial loading before the drive torque is transmitted to the load through the torsionally flexible elastic ring. Moreover, if the torsionally flexible elastic ring should fail through age or excess loading the coupling provides no drive connection between the driving shaft and the driven member.
Another isolating device is described in UK Patent No. 2258517. In this case primary and secondary elements are coupled to one another by coil compression springs. In use the springs are compressed and absorb rotational fluctuations of the driving shaft. In order to provide an appropriate isolation characteristic the coil compression springs occupy a significant amount of space. Moreover, at large relative rotational displacements of the primary and secondary elements the springs reach their limit of compression and the individual coils come into contact with one another so that no further compression is possible. At this stage torque is transferred directly through the device without isolation.